The present invention relates to measuring coating thickness, and more particularly, to using eddy current apparatus with a swept frequency oscillator.
When substrate parts are coated with thin layers, it is important to control the layer thickness. When the coating has physical properties, e.g., acoustic impedance or electrical resistivity, which are distinctly different from the substrate, control of layer thickness is done with standard techniques, such as ultrasound or fixed frequency eddy currents. However, in coating some substrate parts, the substrate and coating materials differ only slightly, e.g., the coating of zircalloy tubing with zirconium metal. Thus, the difference in physical properties is slight, which makes determining coating thickness difficult.
It is therefore an object of the present invention to determine coating thickness, especially when the differences in physical properties of the coating and the substrate are small.